The present invention concerns a twin hydrocyclone used for instance in cellulose and paper mills for purifying pulp suspension, said twin hydrocyclone consisting of two hydrocyclones mounted with their larger diameter ends against each other, said hydrocyclones having one common infeed connector and one common purified fraction discharge connector.
Hydrocyclones, or vortex purifiers, are commonly used in cellulose and paper industry for separating various dirt particles from fiber suspensions, such as sand, bark particles, sticks, stubs of branches and metal chips. The fiber suspension to be purified is conducted under pressure into the hydrocyclone through a tangential infeed connector, whereby the suspension is set in a rapid helical rotary movement, the constituents of the fiber suspension with different specific gravity and shape being separated by action of the centrifugal force created by this movement. As the fiber suspension proceeds along a helical path towards the apex of the cone, the constituents with higher specific gravity, such as sand, are flung to the outer circumference of the rotational movement, close to the wall of hydrocyclone. The impurities concentrated in a layer flowing on a helical path along the wall towards the apical aperture of the cone emerge from the hydrocyclone through the apical aperture of the cone as reject fraction. The fiber suspension purified from dirt particles constitutes a helical flow having a pitch opposite to that of the afore-mentioned helical flow generated in said feeding event, and it emerges through the central connector of the cylindrical separating chamber opposite to the cone.
The twin hydrocyclone also operates on the principle just mentioned. The twin hydrocyclone comprises two separate hydrocyclones fixedly joined by their ends adjacent to the cylindrical separating chamber so that feeding of the hydrocyclones and the withdrawing of the accepted fraction have been connected. The principle of the twin hydrocyclone is readable in the Finnish Pat. No. 56868.
Nowadays it is quite commonplace that hydrocyclones are made of a synthetic material by die-casting the synthetic mix, heated to fluid state, in a dimensionally accurate negative mold, in which the synthetic mass forms the object itself as it cools. When this method is used, the body has to be given such shape that the wall thicknesses of the finished object are uniform and all material concentrations are avoided. Hereby, the cooling of the mass will be uniform, and the object that is produced will exactly retain its shape in the cooling phase, and there will also be no residual stresses in the object which might later during use, together with the operating load acting on the object, result in breakage of the object.
Twin hydrocyclones of prior art have the drawback of complex design, and division of the feed flow which causes disturbances in the feed flow in view of the hydrocyclone's operation. Moreover, in the twin hydrocyclone designs of prior art, the shape and dimensions of the hydrocyclones are such that their manufacturing by the modern methods described in the foregoing cannot be contemplated.
Endeavors have been made to eliminate the disturbances caused by the deficient feeding event in the designs of prior art, by increasing the velocity of the feed flow, with the consequence of increased pressure drop and, therefore, higher energy requirements. As a result of the increase of energy costs, these designs of prior art have become costly as to their operating costs.
In the extremely comprehensive and multiple-step trial runs on which the present invention is based has been observed the indisputable effect of the feeding event on the efficiency with which the hydrocyclone separates the minimal impurity fractions having a specific gravity closely similar to that of the fibers, and which have been the most difficult to separate from the fiber suspension. When aiming at top separation efficiency of said impurity fractions which are difficult to separate, the feeding event of the hydrocyclones has to be dimensioned and constructed in proportion to the other dimensions of the hydrocyclones in such manner that not even the smallest flow interfering with the internal operation of the hydrocyclone, or turbulent flow, vortex, etc. will arise.